I have driven to
Disneyland a half dozen times but never to Disney World and I live in Coquitlam BC.
Last year I booked in May on American Airlines for our September trip and it was $240.00 USD each return out of Seattle, our exchange was better last year so it cost under $275.00 CDN each.
This year I booked last month on American Airlines for our trip this September and paid $240.00 USD ($99.00 each way plus taxes) each return out of Seattle, with the exchange $308.00 CDN each.
You need to factor in parking if fly out of Seatac but if you go to one of the hotels near the airport it is around $50.00 USD a week.
To me it isn't worth trying to be crammed into a car with five people for the amount of time that it would take.
Keep us posted I am curious to see if you do drive.